The selection of model ENMs used for the RESAC Center projects is largely driven by the over-riding hypothesis of the RESAC toxicological projects (Projects 1 and 2), that (i) the cellular, molecular and immune reactivities in response to ENMs depend on the specific physicochemical properties of the ENMs;(ii) the physicochemical properties of the ENMs deposited in the lungs critically depend on their interaction with lung lining fluid, which in turn impacts on ENM reactivity with macrophages and epithelial cells;(iii) in turn, this determines their entry, localization, biological activity and fate in the lung. The selection is also based on potentials for exposure and health risks. We plan to study two types of commercially widely-used ENMs: silver and carbon nanomaterials. Their specific physicochemical properties are summarized in Table SCI. Our selection of these two ENMs allows us to compare biological effects in two systems of great technological importance. By preparing well-controlled ENMs, standardized by particle size and aspect ratio, we will be able to move beyond the particular behaviour of a single chemically homogenous system (whether silver or carbon), in order to isolate, more fundamentally, the effects of geometry and chemistry.